1. Field of the Invention
The present invention relates to a signal transmission system for transmission of digital signals between circuit blocks via a signal transmission line, and to the signal transmission line.
This application claims the priority of the Japanese Patent Application No. 2003-281188 filed on Jul. 28, 2003, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
For a high-speed transmission of digital signals from a CMOS differential driver to a receiver via a signal line, the Inventors of the present invention proposed, before the present invention, to have a power/ground wire pair function as a transmission line whose electromagnetic field is almost closed against a complementary signal energy to be supplied to the driver by forming a power wire and grounding wire formed on a wiring circuit board into a pair wire structure formed from parallel wires equal in length to each other and whose coupling coefficient is large (cf. Japanese Published Unexamined Patent Application No. 284126 of 1999 (will be referred to as a patent document 1)), and enabling a high-speed switching of a transistor by providing a circuit to forcibly pump up and down the necessary charge for state transition of the transistor (cf. Japanese Published Unexamined Patent Application No. 2002-124635 (will be referred to as a patent document 2)).
Note here that for transmission of an electric energy, there should basically used two power lines such as the domestic electric power line. The power line has a conductance similar to that of a water pipe, proportional to the thickness of the latter. The reciprocal of this conductance is called “characteristic impedance Zo”.
The physics of the pipe thickness corresponds to an energy stored in an inductance Lo and capacitance Co per unit length of the power line. Since the energy input and output are made more frequently as its frequency is higher, an alternate-current (AC) resistance, namely, an impedance Z, will take lace. The impedance Z is discharged with a time lag and without loss of the energy, differently from an element for conversion of electric energy into heat energy, such as a direct-current resistor and a leakage conductance between pair wires. Thus, the impedance Z is handled as an imaginary number. The impedance Z is given by the following equations (1) and (2):Z=jωLo  (1)Z=(1/jωCo)  (2)
As shown in the above equations (1) and (2), since two elements are coexistent per unit length in the line, the mean square of them is a characteristic impedance Zo of the line, which is given by the following equation (3):Zo=√jωLo/jωCo=√Lo/Co  (3)
As shown in the above equation (3), three terms including the imaginary number j, angular frequency ω and unit length are eliminated and thus the characteristic impedance Zo will be a real number not defined in length and which does not depend upon any frequency. There is arisen a special physical concept that both a short line and infinitely long one will be equal in characteristic impedance Zo to each other. Briefly, the characteristic impedance Zo determines only the front-end diameter of the line.
As a universal misunderstanding in the domain of the lumped-element circuit, it has been said heretofore that since the transmission line is an inductance-capacitance (LC) network, the RC (resistance-capacitance) delay problem cannot substantially be avoided unless the LC network is considered from the standpoint of a distributed-element circuit. However, the transmission line belongs to the field of electromagnetic phenomenological physics, quite different from a field in which the RC delay is involved. It will be discussed herebelow that the RC delay problem can be solved for the transmission line.
A distributed-element circuit (having the long-distance wiring thereof defined in length) is differentiated from a lumped-element circuit (having the wire length in a negligible range) as given by the following equation (4) (cf. “Silicon Technology”—Feature of the Problems and Outlook of the Ultra High-Speed Multilayer Wiring Techniques—Journal of Applied Physics, Japan Society of Applied Physics, Japan, No. 15, Feb. 18, 2000 (Yamagami Clubhouse, Higashiyama (will be referred to as a non-patent document 1)):Lcritical=λ/40=co√μr∈r/40fclock  (4)where co is a speed of light in vacuum, μr is a specific permeability, ∈r is a specific dielectric constant and fclock is a highest frequency of a clock pulse flowing through the wire.
The above equation (4) defines the relation between the wavelength λ of a sine wave and the wire length Lcritical.
The factor ( 1/40) in the equation (4) will be explained below concerning its meaning with reference to FIG. 1.
As shown in FIG. 1, a digital (pulse) signal is a complex wave fcombine including a fundamental wave f1 and its harmonics f2, f3 . . . . Addition of the harmonic f3 having a frequency three times higher than that of the fundamental wave f1 and harmonic f5 having a frequency five times higher than that of the fundamental wave f1 forms an approximate pulse, and addition of the harmonics f7, f9 and f11, having frequencies seven times, nine times and eleven times, respectively, higher than that of the fundamental wave f1 provides a nearly complete pulse. In other words, the pulse can be said to be a mixed wave including up to a harmonic of a sine wave one order of magnitude higher than the pulse frequency thereof. Therefore, for a pulse of 1 GHz, it is necessary to take up to a harmonic of 10 GHz into consideration. Like a tuning fork, the resonance leads to a minimum resonant frequency equal to a quarter of a wavelength (that is (¼)λ).
Therefore, for transmission of a pulse of 1 GHz in frequency, a lumped-element circuit can conventionally be designed to have a length of up to a quarter of the wavelength of a pulse of 10 GHz, namely, to a length less than 1/40 of the wavelength plus a safety length. This circuit length, by which a distributed-element circuit and lumped-element circuit is differentiated from each other, is defined as a wire length Lcritical. Namely, a circuit having a length of more than ( 1/40)λ should be a distributed-element circuit, namely, a transmission circuit.
As an example of a conventional driver-receiver circuit with a global wire whose length cannot be neglected, a single-ended digital signal transmission circuit 300 is illustrated in FIG. 2.
Although there is shown only a single signal line in FIG. 2 for the simplicity of illustration, the digital signal transmission circuit 300 actually needs two such lines for transmission of an electric energy according to the physical principle. A grounding wire not intentionally formed for reference, or a power line, serves as the second signal line.
In the single-ended digital signal transmission circuit 300, a signal line 311 led out from a driver 310 is paired with a grounding wire 312 to form a signal transmission line 315 via which a complementary signal will be transmitted from the driver 310 to a receiver 320 (cf. “Measurement Evidence of Mirror Potential Traveling on Transmission Lines” by Otsuka, et al., Technical Digest of 5th VLSI Packing Workshop of Japan, pp 27–28, December, 2000 (will be referred to as a non-patent document 2) and “Stacked Pair Wire” by Kanji Otsuka and Tadakazu Suga, Journal of Japan Society of Electronics Packaging, Vol.4, No. 7, pp 556–561, November, 2001 (will be referred to as a non-patent document 3)).
Also, as examples of a conventional differential digital signal transmission circuit an constructional example of a CML (current mode logic) differential transmission circuit 400 is illustrated in FIG. 3 and a constructional example of an LVDS (low voltage differential signaling) differential transmission circuit 500 is illustrated in FIG. 4.
In the differential digital signal transmission circuit 400 (500) constructed as shown in FIG. 3 (4), a complementary signal is transmitted from driver 410 (510) to a receiver 420 (520) via a signal transmission line 415 (515) paired with a grounding wire.
The differential digital signal transmission circuit is said to be suitable for high-speed data transmission, and recently it is frequently used for high-speed transmission of differential signals.
Note here that the signal transmission circuit using a pulse clock having a frequency included in the GHz frequency band has the wire length thereof limited because the wire cannot be long against an RC delay and loss and a dielectric loss. On the other hand, a loner wire is more and more important for communications between functional blocks of a signal transmission circuit. For example, a LAN cable formed from metal conductors is required to assure a signal transmission at a rate as high as 10 Gbps and 100 Gbps. Signal transmission at a rate higher than 10 Gbps over a distance shorter than 100 meters cannot be done by the metal LAN cable but can be done by the optical cables available in the year 2003.